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dripping along vent stack, 2d floor. condensation?

3 year old house. There was dripping through the drywall ceiling of the 1st floor. Opened the ceiling and it is dripping from a toilet stack. I opened the wall at the baseboard level on the second floor right above it, and it turns out there is water dripping along one side of a toilet PVC vent stack running above it , but only within the 2nd floor wall (not sure how far up because i haven't torn up all the drywall). In the attic there is 4 ft of uninsulated PVC vent pipe which goes out the roof. Also running in the attic and tied in to the stack is a smaller PVC which seems to come from the shower. These pipes on the attic are always dry--even above where the toilet stack goes through the 2d floor ceiling all the insulation around it was dry. So is there condensation occuring along the PVC stack at some point under the ceiling? If so why, and what can I do? I thought about insulating the pipes in the attic. I am in Chicago so it is very cold. The attic is unheated. There is no correlation with rain.